A typical flight profile for a civil aircraft can be divided into three main phases of flight—takeoff/climb (in which the aircraft is ascending rapidly to reach a cruise altitude), cruise (in which the aircraft stays at an approximately constant relatively high altitude) and descent/landing (in which the aircraft descends from the cruise altitude to land at the destination).
The optimum altitude for the cruise phase (i.e. the altitude that will result in minimum fuel burn for a given flight) is dependent on several factors. Generally it is found that higher cruising altitudes are favourable with respect to fuel consumption per air-mile due to drag reduction brought about by reduced air density, and increased engine thermal efficiency brought about by reduced air temperature at high altitudes. Tailwinds and headwinds must also be taken into account. On the other hand, the increased engine power settings required to reach these high altitudes result in increased maintenance and fuel costs. These conflicting parameters produce a defined optimum cruise altitude for any given set of mission parameters (such as leg length, aircraft weight and aircraft type). The optimum cruise altitude may increase as the aircraft burns fuel, and thus becomes lighter. Ideally, it would be desirable to allow the aircraft to climb gradually during cruise as the aircraft becomes lighter (known as “cruise climb”).
However, the ability of an aircraft to gradually reach higher altitudes is limited by the requirements of Air Traffic Control (ATC), which often clears aircraft into blocks of airspace 1000 feet high in altitude, requiring two such free blocks whenever an aeroplane is other than at a defined flight level (e.g. at an integer number of thousands of feet), such as when the aircraft is ascending or descending. ATC generally therefore requires the ascent to be made relatively quickly, to avoid the possibility of collisions between aircraft, and to clear one of the blocks in as short an amount of time as possible. This means that cruise-climb, though potentially desirable to the individual operator, is generally not permissible when flying in controlled airspace within the current ATC system. Currently, when in controlled airspace, aircraft ascend between flight levels at a substantially constant airspeed (known as step climb). In these circumstances, airspeed is normally measured in terms of one of Mach number, Equivalent airspeed (EAS) or Calibrated Airspeed (CAS). Aircraft speed is maintained during the ascent by increasing engine thrust. However, this limits the maximum attainable altitude, since ATC normally requires the aircraft to be capable of a step climb from any current altitude. In any event, in a modern aircraft, such aircraft manoeuvres may be carried out by an aircraft Flight Management System (FMS) or Autopilot.
Consequently, it is desirable to have an FMS which is capable of executing climbs to a higher altitude, while conforming to ATC requirements, and reducing fuel burn and engine wear. The present invention provides a computer implemented method of controlling an aircraft and an aircraft control system which seeks to provide such a system.